tests/fs/fa_api: Fix order of checks and broken zassert messages

The commit fixes three things:
 - the order of zassert_* checks has been often incorrect, and some
   checks of values have been done after some other operations has
   been performed;
 - some operations have been missing checks at all;
 - most of zassert_* messages have been given ret parameter to be
   printed but it has been missing from format string.

This addresses:
Coverity CID :215714
GH Issue: #31668

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>